Metro has been put on notice to make safer more than a third of its trains, which are suffering age-related problems including doors that can be forced open while moving, repeated break-ins of drivers' cabins by vandals and cracking beneath carriages.

The safety problems are unique to Metro's Comeng trains, many of which have been running for 30 years and are due to reach the end of their design lives between 2017 and 2024.

Footage emerged on Thursday of a 17-year-old boy jumping out of a Sunbury-bound train travelling at 40km/h, slamming onto the platform but escaping serious injury. His friend had forced the door open.

Metro has been told by the director of Transport Safety Victoria (TSV) the trains will be taken out of service if Comeng doors have not been fixed by 2017 and made impossible to force open. ''We put a condition on Metro's accreditation last year to not allow these trains to be run after 2017 unless they've got this modification,'' said Alan Osborne, director of TSV.

Mr Osborne said the cost of updating the trains would be in the millions but it was essential work.

Meanwhile, Fairfax Media has seen images of several drivers' cabins in Comeng trains that have been broken into and vandalised.

A Metro train driver said in an email that vandals broke into the cabins with ease because of badly worn locks.

Metro responded that it spent more than $10 million a year dealing with vandalism, including graffiti, rock throwing and smashed train windows.

''As security around the network has increased over time, vandals have become more sophisticated,'' spokeswoman Larisa Tait said.